Changes in the lattice constants of thin-film LiCoO2 cathodes at the 4.2 V charged state

Abstract

The lattice constants of thin-film Li1-xCoO2 Cathodes at the 4.2 V charged states were influenced by various deposition conditions. Li1-xCoO2 thin films [yielding a strong (003) texture] on a Pt or Au current collector, which were unheated during sputtering deposition and ex situ annealed, showed negligible lattice expansion at 4.2 V during the first charge. This is in contrast to the Li1-xCoO2 powders exhibiting similar to3% c axis expansion at x congruent to 0.5 (from similar to14.05 to similar to14.45 Angstrom). The total energy of the constrained Li0.5CoO2 lattice (0% c axis expansion) obtained by a pseudopotential total-energy calculation was slightly higher than that of the relaxed one by similar to1.0 eV per 12 Li0.5CoO2 (or similar to80 mcV/Li0.5CoO2), indicating no difficulty of limited lattice expansion during the first cycle. However, splitting of the (009) diffraction peak was observed at 4.2 V as cycling proceeded: one has a lattice constant c of 14.01 +/- 0.05 Angstrom as LiCoO2 before charging, and the other has a lattice parameter of 14.40 +/- 0.05 Angstrom, which is similar to the Li0.5CoO2 powders. In contrast, the lattice constants c of the Li1-xCoO2 thin films deposited at different conditions [yielding a weak (003) texture] expanded when first charged to 4.2 V, which is similar to that observed in the powder geometry.